The present invention relates to a process for preparing dialdehydes, and more particularly to a process for preparing dialdehydes in high productivity in the presence of a catalyst of silver particles specially prepared to prevent them from aggregation.
There have hitherto been known processes for preparing dialdehydes by subjecting glycols to oxidation-dehydrogenation in the presence of a silver catalyst, as disclosed in, for instance, Japanese Unexamined Patent Publication No. 103809/1979, No. 203024/1982 and No. 59933/1983, and the like. Since the silver catalyst gives higher conversion of glycols and higher selectivity into dialdehydes than other catalysts do, it is most expectd that the silver catalyst be put to practical use in industrial production.
Although the silver catalyst has excellent selectivity, many problems have to be solved until it can be used in practical operation, such that (1) the high selectivity is incompatible with the high conversion, (2) the conversion is lowered due to shrinkage with the passage of time or aggregation of the catalyst, and the like.
When the production of dialdehydes using the silver catalyst is carried out at a higher temperature in expectation to obtain higher conversion, the shrinkage and aggregation of the catalyst further rapidly progress to result in rapid increase in pressure drop and generation of clearance between the reactor wall and a catalyst bed in an early stage of reaction. Thus, once obtained higher conversion quickly deteriorates in short period of time and the catalyst used in such a way can no longer continue efficient reaction and becomes impossible to reuse. In other words, the catalyst of the silver particles has a very short life.
In order to solve the above defects, there are some proposals as to processes, in which a silver catalyst is also used, to prepare formaline from methanol. For instance, it is proposed to use a silver-gold alloy catalyst in Japanese Unexamined Patent Publication No. 112806/1979, but this proposal is not practical from the point of view of the total cost required for raw materials of catalyst and recovery, and the like. Also, Japanese Unexamined Patent Publication No. 133214/1976 proposes a process using silver particles having a specific particle size (0.01 to 10 .mu.) or a specified range of specific surface area (3 to 30 m.sup.2 /g), Japanese Unexamined Patent Publication No. 13307/1975 proposes a process wherein silver particles with different particle size are packed in a column to form three or more layers in a certain proportion, Japanese Unexamined Patent Publication No. 33428/1980 proposes a process wherein a net of a metal such as silver or copper is located in a middle of a catalyst layer.
The above proposals in the formaline production process also produce good results in the production of the dialdehydes to a certain extent, but none of them can be substantial solutions to the aforementioned problems. Generally, it is required in commercial plants to start and stop the reaction repeatedly from various operational reasons or troubles. But, by the methods according to the above proposals, if the inner temperature of the reactor decreased to about 300.degree. C. due to interruption of reaction, the catalyst layer is no longer usable because of cracks generated in it and the clearance between the reactor wall. Consequently, it is required to replace the used catalyst with new one. However, it is not easy to take it out from the reactor because the used catalyst is strongly aggregated in the reactor.
An object of the present invention is to provide processes for preparing dialdehydes from glycols in a high conversion of glycols and a high selectivity into dialdehydes, avoiding the above-mentioned problems.
This and other objects of the present invention will become apparent from the description hereinafter.